Transcranial magnetic stimulation (TMS) to the left dorsal-lateral prefrontal cortex (DLPFC) can be useful in the treatment of depression, and the Neuronetics(R)' Neurostar TMS protocol was approved in October of 2008 by the Food and Drug Administration for therapy of certain forms of medication-resistant depression. However, clinical responses are heterogeneous and effect size can be limited. One factor known to contribute to this response variability is differences in the specific site of stimulation in and around the DLPFC. Recent evidence from our lab suggests that the efficacy of different DLPFC targets is related to the connectivity of each target site with deeper limbic regions, specifically the subgenual cingulate. Based on these findings, we have proposed a novel connectivity-based targeting approach to identify the optimal stimulation site in individual patients to maximize antidepressant response. The goal of this project is to empirically validate this approach in actual patients undergoing TMS for depression. Patients referred for treatment by their psychiatrist and found eligible for the FDA-approved Neurostar TMS protocol will be eligible for the study. Participants will undergo an MRI scan including sequences specific to resting state functional connectivity MRI (rs-fcMRI) prior to a four week TMS treatment course (daily sessions Monday to Friday on four consecutive weeks) using FDA approved parameters. The site of TMS administration in each patient will be defined according to the FDA approved Neurostar protocol, but recorded with a noninvasive stereotactic registration system. Clinical antidepressant response to the TMS treatment paradigm will be assessed using the Hamilton Depression Rating Scale (HDRS). Upon completion of the TMS treatment course, clinical response (change in HDRS) will be evaluated as a function of the functional connectivity of the stimulation site as characterized by rs-fcMRI. Our hypothesis is that patients with better clinical response (greater change in HDRS) will show stronger functional connectivity between the stimulation site and deep limbic regions, especially the subgenual. Further, we hypothesize that patients with better clinical response will show a closer approximation between their actual stimulation site and their optimal stimulation site identified with our connectivity-based targeting technique. Should these hypotheses prove correct, the results would lend important insight into the antidepressant mechanism of TMS in depression and provide the foundation for a larger randomized clinical trial to better individually tailor TMS and thus improve its antidepressant efficacy across patients.